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Study Charts 2,000 Years of Continental Climate Changes

By Andrew C. Revkin April 22, 2013 5:53 pmApril 22, 2013 5:53 pm

Photo

A chart from a new study of temperature changes over the continents through 2,000 years. The colors denote the extent of warming or cooling (key at right). The bars denote 30-year periods during which the mean temperature was calculated. (Larger version)Credit Nature / Kaufman et al.

A comprehensive new analysis of temperature changes over the continents through 2,000 years has found that a long slide in temperatures in most regions preceded the unusual global warming of recent decades, but with a lot of regional variability and other fascinating details. A National Science Foundation news release has ample background.

The 78-author paper, published Sunday in Nature Geoscience, used a variety of indirect indicators of temperature, from tree rings to pollen grains, to build on other work charting temperature shifts since the end of the last ice age — including the recent Marcott et al paper, explored here, which used seabed sediments to chart 11,000 years of temperatures.

The new paper drew no conclusions about Africa (you’ll see it’s missing from the chart above) because there are too few spots where long climate records accumulate in lakes or caves. (In most other populated regions, instrumental records have covered the last 100 years, but much of Africa remains a data-free zone even now.)

But along with supporting the general picture of a long temperature slide until the modern era’s warming, the analysis reveals fascinating regional variations, including these:

The Arctic was also warmest during the twentieth century, although warmer during 1941–1970 than 1971–2000 according to our reconstruction….

In Europe, slightly higher reconstructed temperatures were registered in A.D. 741–770, and the interval from A.D. 21–80 was substantially warmer than 1971–2000. Antarctica was probably warmer than 1971–2000 for a time period as recent as A.D. 1671–1700, and the entire period from 141–1250 was warmer than 1971–2000.

I was in touch with Darrell S. Kaufman of Northern Arizona University, one of two lead authors, just before he headed to Alaska’s Brooks range for three weeks of fieldwork. He sent a note for Dot Earth introducing the study, along with a long, helpful “frequently asked questions” sheet. Here’s Kaufman’s guest post:

Dot Earth readers will be interested in a new study published online today in the journal Nature Geoscience. The article, “Continental-scale temperature variability during the last two millennia,” is the most comprehensive evaluation to date of temperature change on Earth’s continents over the past one to two thousand years. The study was authored by the Past Global Changes (PAGES) 2k Consortium, a group of 78 experts from 24 countries.

By investigating the regional-scale patterns of past climate variability, the study helps to understand and quantify the processes that cause climate to change. This is important as we prepare for the full range of future climate changes, whether they are due to anthropogenic or to natural factors.

Previous studies have focused on hemispheric or global-scale temperature reconstructions, which are useful for understanding overall average conditions, but can overlook important differences at regional scales. In particular, the new synthesis clarifies the temperature history for the Southern Hemisphere continents by including temperature reconstructions from Antarctica, Australasia and South America.

The synthesis highlights the most prominent features of seven new and recently published continental-scale temperature reconstructions. Most of proxy temperature records are from tree-ring measurements, but additional evidence comes from glacier ice, speleothems [stalagmites, stalactites and related mineral formations], corals and lake sediments.

My co-authors and I found that, prior to the late 19th century, temperatures had been cooling in nearly all of the continental-scale regions. The long-term cooling in the Arctic can be attributed to the effect of orbital cycles and is known from previous studies. [I covered one, by a team including Kaufman, in 2009.]

Finding a similar trend in the Southern Hemisphere was somewhat unexpected because the effects of the orbital changes should be opposite there.

The long-term, pre-20th-century cooling trend is among the most prominent features of the regional temperature reconstructions. We probed the data using several statistical approaches and came up with the same result. When averaged across the regions, the long-term cooling trend in the PAGES 2k reconstructions agrees with the global cooling trend determined recently by Marcott and coauthors(Science 339: 1198) in their study of temperature changes over the past 11,000 years, which was based primarily on marine sedimentary records.

The long-term cooling trend was only slight, between about 0.1 and 0.3°C per 1000 years, depending on the region. It was caused by natural factors that likely continued through the 20thcentury, making the recent warming more difficult to explain without the impact of increased greenhouse gases.

We also found that temperatures in some regions were higher in the past then they were during the late 20th century and that, the longer the individual site record, the more likely it was to show prior warm intervals, which is consistent with the long-term cooling trend. In Europe, for example, the average temperature between AD 21 and 80 was warmer than during AD 1971-2000. But temperatures did not fluctuate uniformly among all regions at multi-decadal to centennial scales. For example, the transition to colder regional climates between AD 1200 and 1500 is evident earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere.

The study focused on the most prominent features of the continental-scale temperature reconstructions. The data assembled for this synthesis, and the temperature reconstructions derived from them will no doubt be analyzed using alternative approaches to reveal other patterns and address further research questions. For example, all reconstructions, except for North America, are resolved at annual scale, but our analysis only considered averages over 30-year periods.

The PAGES 2k Consortium, in coordination with NOAA Paleoclimatology, has tabulated all of the data used for the continental-scale reconstructions in a uniform and user-friendly format. This data product will be useful in future studies, including as a benchmark for comparisons with climate-model simulations that attempt to account for both anthropogenic and natural factors in projections of future climate.

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By 2050 or so, the human population is expected to pass nine billion. Those billions will be seeking food, water and other resources on a planet where humans are already shaping climate and the web of life. Dot Earth was created by Andrew Revkin in October 2007 -- in part with support from a John Simon Guggenheim Fellowship -- to explore ways to balance human needs and the planet's limits.